Is it possible to run a shared php-pool while keeping security?

[Sylvia]

It will need to spawn the pool, if you're on ultra-cheap-ultra-low-powered
vps, this can be slow, because of slow cpu, low ram and very small disk
cache portion which belongs to your vps, on a dedicated machine with 2 GB
ram i dont notice any delays with spawning fpm pools on demand. Anyway i
dont see much options for you with 128 Megs,  even better configs can be
slow, depending on the vps provider and hypervisor they use.

Additional suggestion to you - remove PHP extensions which are not needed

extension=hash.so
extension=session.so
extension=dom.so
extension=xml.so
extension=mysql.so
extension=mbstring.so
extension=zlib.so
extension=gd.so 
extension=curl.so
extension=json.so
;extension=bcmath.so
extension=exif.so 
extension=mcrypt.so
extension=simplexml.so
;extension=pdo_mysql.so
extension=sysvmsg.so
extension=tokenizer.so
extension=ctype.so
extension=mysqli.so
extension=iconv.so
extension=gmp.so
extension=openssl.so
extension=gettext.so
extension=zip.so

anything except that list is most likely just hogging your RAM )

Make sure to install a PHP opcode cache (not less than 32M), it will hog its
own piece of RAM but will decrease RAM consumption for processing scripts,
i.e. Wordpress 28 Mb to 4 Mb, eaccelerator still can swap to disk
precompiled scripts, therefore (at my look) it will be the best choice (you
will need master branch from GIT for PHP 5.4)

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,230730,230745#msg-230745