Problems with slow update

Hi,

on a single MongoDB 2.0.2 instance I have collection with documents
like this:

 db.assortment.findOne()
{
	"_id" : ObjectId("4f3279b85e53cee38eefa2fa"),
	"pk" : "2j1IRLU8g0kSvJv9CBYV58-1",
	"code" : "323948-40",
	"referenceNr" : "TLP LV6",
	"status" : {
		... small subdocument
	},
	"exclusiveCompany" : false,
	"masterCode" : "324948",
	"productManagerCode" : "Y_RINGER",
	"manufacturer" : {
		... small subdocument
	},
	"modulePk" : "SPWUfGcRlHRLSp1csHvW58-1142",
	"mprgPk" : "UdWfxpZ0mHRLSp1csHvW58-1142",
	"prgPk" : "D5sezwJ3nHRLSp1csHvW58-1142",
	"visible" : [
		"iQJtnEnq0rxBxmWbsHvW58-32"
	],
	"visibleIso" : [
		"de.CH"
	],
	"searchable" : [ ],
	"searchableIso" : [ ],
	"localized" : {
		"iQJtnEnq0rxBxmWbsHvW58-32" : {
		... small subdocument
		}
	},
	"property" : {

	},
	"bios" : [ ],
	"context" : [
		... around 50 entries
	],
	"update" : null
}

and the following stats after a fresh import:

 db.assortment.stats()
{
	"ns" : "hardware.assortment",
	"count" : 263089,
	"size" : 1082690256,
	"avgObjSize" : 4115.300358433838,
	"storageSize" : 1247039488,
	"numExtents" : 22,
	"nindexes" : 9,
	"lastExtentSize" : 215420928,
	"paddingFactor" : 1,
	"flags" : 0,
	"totalIndexSize" : 1641332000,
	"indexSizes" : {
		"_id_" : 8985424,
		"pk_1" : 15755152,
		"code_1" : 9770320,
		"exclusiveCompany_1" : 8617504,
		"context_1" : 1476798176,
		"bios_1" : 16262064,
		"searchable_1" : 56921312,
		"visible_1" : 37339792,
		"referenceNr_1" : 10882256
	},
	"ok" : 1
}

When doing an update it takes 60 seconds to update 1211 document:

Wed Feb  8 14:59:22 [conn8] update hardware.assortment query: { pk:
{ $in: [ <1211 values like "DgGf03K9oUQJA5x9CBYV58-1"> ] } } update:
{ $push: { bios: "b_SY96C2vGBKRLSp1csHvW58-26" } } moved:1 69318ms

and after the update the collection stat looks like:

 db.assortment.stats()
{
	"ns" : "hardware.assortment",
	"count" : 263089,
	"size" : 1087657972,
	"avgObjSize" : 4134.1826226105995,
	"storageSize" : 1247039488,
	"numExtents" : 22,
	"nindexes" : 9,
	"lastExtentSize" : 215420928,
	"paddingFactor" : 1.409999999999991,
	"flags" : 0,
	"totalIndexSize" : 1645444528,
	"indexSizes" : {
		"_id_" : 8985424,
		"pk_1" : 15746976,
		"code_1" : 9762144,
		"exclusiveCompany_1" : 8642032,
		"context_1" : 1480133984,
		"bios_1" : 16360176,
		"searchable_1" : 56954016,
		"visible_1" : 37977520,
		"referenceNr_1" : 10882256
	},
	"ok" : 1
}

What I'm doing wrong? A find or count on the very same query object
used for update returns within 50-300 ms so why does it take that long
to update 1211 documents?

Addiotional Info:

In the time the update runs, the following lines appear in log:

Wed Feb  8 14:58:13 [conn8] old journal file will be removed: ../data/
local/journal/j._33
Wed Feb  8 14:59:19 [conn8] old journal file will be removed: ../data/
local/journal/j._34
Wed Feb  8 14:59:19 [conn8] old journal file will be removed: ../data/
local/journal/j._35

Does this mean MongoDB needs 400 Megabytes of journaling to update
1211 documents?

Greetings,
Gerhard Balthasar