It sounds to me like you are confusing incompetent admins with bad products. You absolutely do not need IBM service people to install DB2 and have it perform like you want.
A mainframe is not a PC. You can not just buy an off-the-shelf mainframe, load some software on it, and expect it to perform as you want. Every mainframe is built to customer order, and there are literally millions of possible configurations. This includes not only obvious stuff like how much memory and how many cores do you want, but workload-dependant stuff like what speed should the processors run at, how many cores are dedicated to running IO operations, how many to DB2 functions, how many to Java functions, etc. It is up to the customer to understand their workload and configure the machine appropriately.
Then, once the machine is installed, there is more configuration to be done (by the customer). You must create your IOCDS to properly balance your expected IO over all of the IO domains. You must dedicate enough channel paths to your busiest devices, etc.
So, it is no surpise that someone who does not know what they are doing can make an unholy mess of things and wind up with an extremely poor performing system. Yes, IBM will help you fix your mess. No, that does not mean IBM produced a shoddy product.
Then, you need to define your LPARs. What percentage of the resources does each LPAR get? Do the LPARs have caps on them, etc?