Extreme slowness when using Zen or a subtheme

Currently building a new site which needs to support Hebrew and RTL layout. I'm trying to use Zen 2.x as it's supposed to have better RTL support.

I'm experiencing extreme slowness on every page load which doesn't seem to be happening when using Garland. This happens even when disabling all contributed modules. However, it does NOT reproduce itself when installing a clean D6 with Zen.

Any ideas will be welcome.

I have the following modules enabled:

auto_nodetitle
block
blog
computed_field
content
content_copy
content_permissions
content_profile
content_profile_registration
content_taxonomy
content_taxonomy_options
content_taxonomy_tree
ctools
date
date_api
date_locale
date_popup
date_timezone
dblog
delegator
devel
draggableviews
draggableviews_cck
email
emfield
emvideo
fieldgroup
filefield
filter
fivestar
flag
help
imageapi
imageapi_gd
imagecache
imagecache_ui
imagefield
image_fupload
image_fupload_imagefield
link
locale
menu
nice_menus
node
nodecomment
nodeforum
nodereference
number
optionwidgets
panels
panels_export
panels_tabs
path
pathauto
php
plus1
popups
revision_moderation
statistics
system
tabs
taxonomy
taxonomy_image
taxonomy_manager
text
token
translation
trigger
upload
user
views
views_content
views_export
views_ui
votingapi
wysiwyg