Informix dual processor scalability

Does anyone have any info on the following problem? We are using Informix version 7.10.UC1 for Intel UnixWare 2.01 on single processor and SMP machines. Using Informix on 1 processor we achieve good performance. If we switch to 2 processors, then our performance actually degrades. We have been given some advice on tuning the system for dual processor machines and we have tried different settings for virtual processors and processor affinity. We managed to get an improvement, but the best scaling we can get is about 1.25. If we move to a quad processor system the problem goes away and we approach expected performance levels. Has anyone else experienced this?