The Intel Core i5-10400H is a fast processor for laptops with four cores based on the Comet Lake-H series (4th generation of Skylake architecture). The processor clocks at between 2.6 and 4.6 GHz (up to 4.4 with 4 cores) and can execute up to eight threads simultaneously thanks to Hyper-Threading. According to Intel, the CPU is manufactured in 14nm (14nm++) process.
The Comet Lake architecture is similar to Coffee Lake and offers the same features and is produced in the same 14nm process. Other than the improved clock rates, the memory controller now also supports faster DDR4-2933 RAM. More information on Comet Lake and all the models and articles on it can be found here.
The Core i5-10400H supports the management features vPro, TXT and SIPP.
The performance should be slightly better than the old Core i7-8809G (Quad-Core 3.1 - 4.2) which was the fastest mobile quad core by Intel.
The integrated Intel UHD Graphics 630 iGPU is supposed to offer a similar performance as the UHD630 in the Core i5-9300H. As a low-end solution it will probably only run current games smoothly at reduced details - if at all.
Intel specifies the TDP with 45 watts and therefore the i5 is only suited for big laptops with good cooling solutions. Using cTDP-down, the CPU can also be configured to 35 Watt resulting in a reduced performance.