High csrss.exe utilisation on XP with legacy software

[Robert William]

I'm serving RealVNC Free Edition 4.1 from an XP32 SP3 box. When I run legacy Turbo Pascal programs from the client the csrss.exe utilisation jumps up to 100% on every keystroke (or even if the application is put in foreground) for several seconds and sometimes as long as a minute or 2. This stalls things and makes the session unuseable. The Turbo Pascal programs don't generate much screen traffic as it is mostly file shuffling so I don't understand why csrss is getting so much utilisation.
Anyone understand this and/or have a fix or workaround? It's a big job to redo the legacy stuff. (Clients are XP or W7 machines and all demonstrate the problem).
Thanks.