If it works fine on some machines but not others, make sure the machine it is running slower on has the same indexing as the faster one (it is surprising how often the indexes don't get moved to the other machine) and update the statistics.
It can also be much slower if the hardware is less robust on the slower machine.
Profiler and execution plans can also help identify bottlenecks and performance problems.
Check for deadlocks as well, often code that works fine by itself can conflict with something else the production server is running at the same time.
Also many developers create code on a machine that has much less load and far less data than the eventual production machine and then are surprised when the code is slow on prod. If you didn't have a full data set on the dev machine, you are running serious risks of writing poorly performing code. If you don't do load testing, maybe you should.
And why develop using Express if the final will not be in Express, buy Developer version and have all the tools you need to tune and monitor while developing.
Are other applications running on the machine that is slow? It is generally a poor practice to run anything except SQL Server on a machine as SQl Server will want all the memory of the machine.