var t = db.slow_truncate;

function insert() {
    for (var i=0; i<100*1000; i++) t.insert({_id: i});
}

print("insert: " + Date.timeFunc(insert));
print("remove: " + Date.timeFunc(function() {t.remove({})}));

t.drop(); // start next phase clean

print("insert: " + Date.timeFunc(insert));
print("trunc: " + Date.timeFunc(function() {t.runCommand('emptycapped')})); // does a truncate

Thread 5 (Thread 0x7f65d7847700 (LWP 13329)):
#0  0x00007f65d5f351b3 in select () from /usr/lib/libc.so.6
#1  0x0000000001476581 in __wt_sleep (seconds=<optimized out>, micro_seconds=<optimized out>) at src/third_party/wiredtiger/src/os_posix/os_sleep.c:23
#2  0x0000000001420cf3 in __wt_page_in_func (session=0x2dad640, ref=0x56bf4e0, flags=0) at src/third_party/wiredtiger/src/btree/bt_page.c:188
#3  0x0000000001437773 in __wt_page_swap_func (flags=0, session=<optimized out>, held=<optimized out>, want=<optimized out>) at src/third_party/wiredtiger/src/include/btree.i:1155
#4  __wt_row_search (session=0x2dad640, srch_key=0x421d158, leaf=0x0, cbt=0x421d080, insert=0) at src/third_party/wiredtiger/src/btree/row_srch.c:297
#5  0x0000000001418bb7 in __cursor_row_search (cbt=<optimized out>, insert=0, session=<optimized out>) at src/third_party/wiredtiger/src/btree/bt_cursor.c:239
#6  __wt_btcur_search (cbt=0x421d080) at src/third_party/wiredtiger/src/btree/bt_cursor.c:310
#7  0x000000000144ae09 in __curfile_search (cursor=0x421d080) at src/third_party/wiredtiger/src/cursor/cur_file.c:200
#8  0x0000000000e7770b in mongo::WiredTigerRecordStore::deleteRecord (this=0x2c0f800, txn=0x7f65d7846718, loc=...) at src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp:362
#9  0x0000000000e798a4 in mongo::WiredTigerRecordStore::truncate (this=0x2c0f800, txn=0x7f65d7846718) at src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp:723
#10 0x0000000000984cf8 in mongo::Collection::truncate (this=0x2a14400, txn=0x7f65d7846718) at src/mongo/db/catalog/collection.cpp:531
#11 0x0000000000a298e5 in mongo::EmptyCapped::run (this=<optimized out>, txn=0x7f65d7846718, dbname=..., cmdObj=..., errmsg=<error reading variable: Cannot access memory at address 0x0>, result=..., fromRepl=false) at src/mongo/db/commands/test_commands.cpp:230
#12 0x0000000000a76b2e in mongo::_execCommand (txn=0x0, c=<optimized out>, dbname=<error reading variable: Cannot access memory at address 0x0>, cmdObj=<error reading variable: Cannot access memory at address 0x7>, queryOptions=-679194984, errmsg=<error reading variable: Cannot access memory at address 0x0>, result=..., fromRepl=<optimized out>) at src/mongo/db/dbcommands.cpp:1326
#13 0x0000000000a77d41 in mongo::Command::execCommand (txn=0x7f65d7846718, c=0x2a1f8a0, queryOptions=0, cmdns=<optimized out>, cmdObj=unonwed BSONObj 36 bytes @ 0x3e99426, result=..., fromRepl=<optimized out>) at src/mongo/db/dbcommands.cpp:1547
#14 0x0000000000a792b6 in mongo::_runCommands (txn=0x7f65d7846718, ns=0x3e99414 "test.$cmd", _cmdobj=..., b=..., anObjBuilder=..., fromRepl=false, queryOptions=<optimized out>) at src/mongo/db/dbcommands.cpp:1621
#15 0x0000000000a79751 in mongo::runCommands (txn=0x0, ns=0x0, jsobj=<error reading variable: Cannot access memory at address 0x8>, curop=..., b=..., anObjBuilder=..., fromRepl=<optimized out>, queryOptions=0) at src/mongo/db/dbcommands.cpp:1651
#16 0x0000000000b4b7c6 in mongo::receivedCommand (txn=0x7f65d7846718, nss=..., client=..., dbResponse=..., message=...) at src/mongo/db/instance.cpp:238
#17 0x0000000000b48f1a in mongo::assembleResponse (txn=0x7f65d7846718, m=..., dbresponse=..., remote=...) at src/mongo/db/instance.cpp:467
#18 0x0000000000862ec6 in mongo::MyMessageHandler::process (this=<optimized out>, m=..., port=0x2c5a490, le=0x2c59fe0) at src/mongo/db/db.cpp:212
#19 0x000000000103af4c in mongo::PortMessageServer::handleIncomingMsg (arg=0x2c5a490) at src/mongo/util/net/message_server_port.cpp:230
#20 0x00007f65d61fe374 in start_thread () from /usr/lib/libpthread.so.0
#21 0x00007f65d5f3c27d in clone () from /usr/lib/libc.so.6