why threadpool implementation is slower than normal threads

I have a code block

for (i=0;i<size;i++) { do job; } 

initially this job task was executed sequentially (as shown above) but later I made multithreaded with normal threads (inner runnable class implementation) like,

for (i=0;i<size;i++) { new threadingclass(some args) } runnable threadingclass { pub void run () { do job; } } 

this worked fine with some thread limit (till system resources were enough) so to avoid resource overloading I implemented same code with standard theadpool implementation (threadpool,executor service and worker thread implementation)

threadexecutor t=new threadexecutor(size) for (i=0 ; i<size ; i++) { t.execute(new threadingclass(some args)) } runnable threadingclass { pub void run () { do job; } } 

now scenario was like,

I wanted to run loop for 25 times (no. of threads), I tried with all 3 implementations

1. sequential : takes 7 min approx
2. normal multithreading : 40 sec
3. multithreading with threadpool (size : 100) : 2 min approx

I am bit confused why normal threading and thredpool implementation timings differ so much and internally also threadpool does not involve much complex logic. any help is appreciated.

Thanks in advance