You should definitely not use integer constraints when you don't need them.
The real question is, do you need to use ga? Think carefully about your problem. If it is smooth with smooth constraints (such as the one you mentioned), then use an Optimization Toolbox™ solver such as fmincon for speed and robustness. If your problem is not smooth, try patternsearch for speed and robustness.
It sounds like you don't need high accuracy for your solution. It is much easier to tune patternsearch than ga; for example, for two decimals of accuracy, set MeshTolerance to 3e-3 or something similar.
MATLAB mathematical toolbox documentation