It's not simply a splash page, there is a lot of additional software that o2 bundle in there, normally it is quite good, however the processor is not very powerful, therefore it can get quite sluggish, if you remove all the "extras" and just leave it to the bare minimum in terms of just the OS it will run much, much quicker.
Getting geeky here, but the processor runs at around 60% of its capability by default, you can increase that and you will notice how much quicker it is.
******just for extra info, PAY AS YOU GO PHONES are locked on o2 but the contract ones are not.*****
Yup ^ we run Voda sims in ours. Yet to get rid of the O2 splash page yet.