The GC runs are legacy and slows things down quite a bit

benchmark                         old ns/op      new ns/op      delta
BenchmarkReplaceAll-8             2942370526     2866418930     -2.58%
BenchmarkUpdateOneChanged-8       7402489        226635         -96.94%
BenchmarkUpdateOneUnchanged-8     7298777        229090         -96.86%
BenchmarkNeedHalf-8               113608416      104483393      -8.03%
BenchmarkHave-8                   29834263       29288220       -1.83%
BenchmarkGlobal-8                 162773699      159269126      -2.15%
BenchmarkNeedHalfTruncated-8      111943400      108235000      -3.31%
BenchmarkHaveTruncated-8          29490369       28945489       -1.85%
BenchmarkGlobalTruncated-8        165841081      149355833      -9.94%

benchmark                         old allocs     new allocs     delta
BenchmarkReplaceAll-8             1054942        1054944        +0.00%
BenchmarkUpdateOneChanged-8       1149           1135           -1.22%
BenchmarkUpdateOneUnchanged-8     1135           1135           +0.00%
BenchmarkNeedHalf-8               374774         374777         +0.00%
BenchmarkHave-8                   151995         151995         +0.00%
BenchmarkGlobal-8                 530042         530063         +0.00%
BenchmarkNeedHalfTruncated-8      374697         374699         +0.00%
BenchmarkHaveTruncated-8          151834         151834         +0.00%
BenchmarkGlobalTruncated-8        530050         530021         -0.01%

benchmark                         old bytes      new bytes      delta
BenchmarkReplaceAll-8             5074294728     5074297112     +0.00%
BenchmarkUpdateOneChanged-8       141048         135097         -4.22%
BenchmarkUpdateOneUnchanged-8     134976         134976         +0.00%
BenchmarkNeedHalf-8               44734813       44759436       +0.06%
BenchmarkHave-8                   11911634       11911138       -0.00%
BenchmarkGlobal-8                 80436854       81609867       +1.46%
BenchmarkNeedHalfTruncated-8      46514673       46588024       +0.16%
BenchmarkHaveTruncated-8          11348357       11348354       -0.00%
BenchmarkGlobalTruncated-8        81730740       79485168       -2.75%

lib/db/leveldb.go

@@ -12,7 +12,6 @@ package db
 import (
 	"bytes"
 	"fmt"
-	"runtime"
 	"sort"
 
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -170,8 +169,6 @@ func globalKeyFolder(key []byte) []byte {
 type deletionHandler func(db dbReader, batch dbWriter, folder, device, name []byte, dbi iterator.Iterator) int64
 
 func ldbGenericReplace(db *leveldb.DB, folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker, deleteFn deletionHandler) int64 {
-	runtime.GC()
-
 	sort.Sort(fileList(fs)) // sort list on name, same as in the database
 
 	start := deviceKey(folder, device, nil)                            // before all folder/device files
@@ -306,8 +303,6 @@ func ldbReplace(db *leveldb.DB, folder, device []byte, fs []protocol.FileInfo, l
 }
 
 func ldbUpdate(db *leveldb.DB, folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker) int64 {
-	runtime.GC()
-
 	batch := new(leveldb.Batch)
 	l.Debugf("new batch %p", batch)
 	snap, err := db.GetSnapshot()
@@ -601,8 +596,6 @@ func ldbWithHave(db *leveldb.DB, folder, device []byte, truncate bool, fn Iterat
 }
 
 func ldbWithAllFolderTruncated(db *leveldb.DB, folder []byte, fn func(device []byte, f FileInfoTruncated) bool) {
-	runtime.GC()
-
 	start := deviceKey(folder, nil, nil)                                                  // before all folder/device files
 	limit := deviceKey(folder, protocol.LocalDeviceID[:], []byte{0xff, 0xff, 0xff, 0xff}) // after all folder/device files
 	snap, err := db.GetSnapshot()
@@ -706,8 +699,6 @@ func ldbGetGlobal(db *leveldb.DB, folder, file []byte, truncate bool) (FileIntf,
 }
 
 func ldbWithGlobal(db *leveldb.DB, folder, prefix []byte, truncate bool, fn Iterator) {
-	runtime.GC()
-
 	snap, err := db.GetSnapshot()
 	if err != nil {
 		panic(err)
@@ -787,8 +778,6 @@ func ldbAvailability(db *leveldb.DB, folder, file []byte) []protocol.DeviceID {
 }
 
 func ldbWithNeed(db *leveldb.DB, folder, device []byte, truncate bool, fn Iterator) {
-	runtime.GC()
-
 	start := globalKey(folder, nil)
 	limit := globalKey(folder, []byte{0xff, 0xff, 0xff, 0xff})
 	snap, err := db.GetSnapshot()
@@ -887,8 +876,6 @@ nextFile:
 }
 
 func ldbListFolders(db *leveldb.DB) []string {
-	runtime.GC()
-
 	snap, err := db.GetSnapshot()
 	if err != nil {
 		panic(err)
@@ -920,8 +907,6 @@ func ldbListFolders(db *leveldb.DB) []string {
 }
 
 func ldbDropFolder(db *leveldb.DB, folder []byte) {
-	runtime.GC()
-
 	snap, err := db.GetSnapshot()
 	if err != nil {
 		panic(err)
@@ -966,8 +951,6 @@ func unmarshalTrunc(bs []byte, truncate bool) (FileIntf, error) {
 }
 
 func ldbCheckGlobals(db *leveldb.DB, folder []byte, globalSize *sizeTracker) {
-	defer runtime.GC()
-
 	snap, err := db.GetSnapshot()
 	if err != nil {
 		panic(err)