latency increases dramatically on same pc when upgrading from 2.4 to 2.7

Although i have very limited knowledge on Linux, i have been driving a Linuxcnc 2.4.6 based on Ubuntu 10.04 on my Amilo 1818 Laptop (Intel Core2 T5300) for several years successfully. Now i though it was time to upgrade to the current system. 2.7 using the live system ISO. But when trying this on the same laptop, the latency increased from 9335/11201 to 153554/128637.

Is there a way to improve this or do i have to stick to the 2.4 Linuxcnc? Due to my limited Linux knowledge and the lack of a test linux system (i won't touch my production system 2.4) i see no way to test other linuxcnc systems as offfered at buildbot.linuxcnc.org because no live systems are offered there.

Thanks for any support.
Regards Uli

Thanks andypugh for your reply.

The 2.7 live system i used for the tests is a complete OS including Linuxcnc 2.7 based on Debian Wheezy.

How can i get a Linuxcnc 2.7 / Ubuntu 10.4. test system? On buildbot.linuxcnc.org in /dists/lucid/2.7-rt/binary-i386 i found a linuxcnc-dev_2.7.8.9.g332beb8_i386.deb file. Seems to be Linuxcnc 2.7 for Ubuntu 10.4. But i dont know how to make a live system (.iso file) for testing from this .deb file (dumb Windows user). So i hope that someone can point me to a Ubuntu 10.4 / Linuxcnc 2.7 live system ready to use. Or propose a completely different solution?

Regards Uli