The Intel Core i5-8259U is a quad-core SoC for notebooks based on the Coffee Lake architecture and was announced in April 2018. Compared to its predecessor Core i5-7267U, the CPU is now a quad-core with Hyperthreading support for the execution of up to 8 threads simultaneously. The base frequency is 2.3 GHz, but the Turbo goes all the way up to 3.8 GHz. It is also equipped with an Intel Iris Plus Graphics 655 GPU with 128 MB eDRAM, a dual-channel memory controller (DDR4) as well as VP9 and H.265 video decoding as well as encoding. The chip is still manufactured in a 14nm process with FinFET transistors.
Compared to faster Core i5 and i7 models with eDRAM and 28 Watts, the i5-8259U only features 6 MB smart cache and the slowest clock of the Iris Plus GPU.
Intel basically uses the same micro architecture compared to Kaby Lake, so the per-MHz performance does not differ. The manufacturer only reworked the Speed Shift technology for faster dynamic adjustments of voltages and clocks, and the improved 14nm process allows much higher frequencies combined with better efficiency than before.
Considering the clocks of the Core i5-8259U, its performance should be between the 15W models Core i5-8350U and Core i7-8550U. The i5-8259U should still be faster than the i7-8550U in practice thanks to the increased TDP, especially during sustained workloads.
The integrated Intel Iris Plus 655 Graphics is the GT3e model of the Kaby Lake GPU (Intel Gen. 9.5). It has 48 Execution Units running at 300-1050 MHz (slowest clock of the 655, the fastest is 1200 MHz in combination with the i7-8559U) and the performance is comparable to a GeForce 930M or 940MX thanks to fast eDRAM cache. However, there aren't any significant improvements compared to the old Iris Plus 650, so modern games can often not be played smoothly or only at the lowest or medium settings, respectively.
Contrary to Skylake, Kaby lake now also supports H.265/HEVC Main 10 with a 10-bit color depth as well as Google's VP9 codec. The dual-core Kaby Lake processors announced in January should also support HDCP 2.2.
The chip is manufactured in an improved 14nm process with FinFET transistors, which improves the efficiency even further. Intel specifies the TDP with 28 Watts, which can be reduced to 23 Watts (cTDP Down) depending on the usage scenario. The TDP is pretty high compared to the common 15-Watt TDP for quad-core processors, but allows a better utilization of CPU and GPU Turbo.