This paper analyzes the statistical convergence of Vendi score and RKE score, which are existing methods for evaluating the diversity of generative models without reference data, and presents a new method to solve the computational cost problem of Vendi score. The existing Vendi score requires eigenvalue decomposition of a high-dimensional matrix, which has limitations in applying it to large-scale datasets. In this paper, we propose $t$-truncated Vendi score that truncates the eigenspectrum, improving it to converge stably even with a limited number of samples. In addition, we show that Nystr om and FKEA approximation methods converge to the asymptotic limit of $t$-truncated Vendi score. On the other hand, we prove that RKE score guarantees universal convergence for all kernel functions. Through experiments, we show that Vendi score using Nystr om and FKEA converges closely to $t$-truncated Vendi score, and analyze the correlation with the diversity of image and text data.